Molecular electronics is the study of charge transport through single molecules or molecular ensembles. The term “molecular junction” may be used to describe a single molecule or a molecular ensemble oriented in parallel between conducting contacts, and may be viewed as the basic component of molecular electronics. Charge transport through molecules has been investigated with techniques such as scanning tunneling microscopy, conducting probe atomic force microscopy (AFM) and vapor deposition of top contacts. Current-voltage measurements on molecular junctions have exhibited phenomena such as rectification and conductance switching. The introduction of molecular electronic components into integrated circuits may be a primary goal of the field.
Numerous approaches have been used to fabricate molecular layers on conducting surfaces, including self-assembly, Langmuir-Blodgett techniques, and the formation of C—C or Si—C irreversible bonds (3.5-4.0 eV) between a substrate and a molecular layer. Irreversible bonding may provide the molecular layer with structural stability during subsequent fabrication and characterization processes, thus reducing the likelihood of molecular damage or metal penetration. Although the resulting layers may be less ordered than self-assembled monolayers (SAMs), irreversible bonding may allow the formation of molecular multilayer structures in which the thickness may be controlled by altering the deposition conditions.
Although significant progress has been made in the field, the ability to fabricate robust junctions with high yields has proven experimentally difficult, primarily due to problems associated with the formation of the second contact. Contact formation through vapor deposition (e.g., metal evaporation) may have several benefits, including the possibility of parallel fabrication, the ability to form contacts with varying work functions, and its compatibility with semiconductor processing. However, vapor deposition may have experimental limitations such as metal penetration through the molecular layer and molecular damage, possibly resulting in behavior characteristic of electronic shorts.
By using spectroscopic techniques, it has been shown that direct evaporation of reactive metals such as titanium (Ti) may result in significant structural damage to the molecular layer. Direct evaporation of noble metals commonly results in partial molecular damage, molecular displacement at the substrate/molecule interface, or penetration between the molecules, for example, in the case of gold (Au). These results may depend strongly on the substrate/molecule bonding characteristics, the type of molecular layer and terminal groups. Fabrication techniques have been developed to mitigate metal penetration and molecular damage, including spin-coating of a conducting polymer contact, indirect evaporation, and the evaporation of copper (Cu).
Vapor deposition of metals onto “soft” materials such as organic electronic materials and molecular monolayers often results in damage to the molecular layer or penetration of metal atoms into the layer. The result can be changes in molecular structure, or “short circuits” caused by metal filaments formed during penetration. This problem has been severe in molecular electronics, since many of the molecular structures are very thin (e.g., less than 5 nanometers) and fragile, and subject to thermal damage during metal deposition. The penetration of metals into such thin films often results in direct contact of the metal to the substrate, bypassing the molecular layer altogether.
Metal deposition onto organic materials is generally avoided in current technology, or the organic layer is sufficiently thick that some metal penetration can be tolerated. Many organic materials simply may not tolerate vapor deposition of metals, often resulting in low device yield or drastically modified device properties.
Conventional metal deposition onto a soft material may subject the material to both kinetic energy and temperature excursions. The heat of condensation of a metal atom is typically 3-4 eV, enough to break bonds and modify structures of organic molecules. As noted above, metal penetration is often a problem, resulting in low device yield. Most methods for metal vapor deposition onto soft targets are inefficient or cumbersome, such as “thermalizing” the metal ions with a low pressure of inert gas.
Thus, significant improvements are needed in the evolution of molecular electronic device technology.